I want to add tower speakers to my stock system and be able to control there volume independently. What's the best way to do this? Can I use the fader or is it easier to get another volume control? I'd like to be able to just run the tower speakers without the others also...

Rrw71- this is a really good idea and your question comes up a lot. From a safety standpoint, I can't stress enough, how important it is to have control of your stereo system. Some people like to have active control of all three zones -- sub, cabin and tower. But at the vary least every tower system should have a way to reduce volume once you drop your rider. Fiddling with your fade on your deck is not a good option.

There are several ways you can add this feature into an existing tower setup, but the easiest is to use a device like a PK-LC1 remote level controller. The part costs 10 bucks on amazon. It remotely mounts on your helm and enables quick and easy control of your tower sound. It would be wired upstream of the amplifier signal path. Very easy to install.

Lastly, it's a good device to have in place so you can blend your tower speakers into the overall sound field while cruising, or ratted up with other boats.

Easy solution is to just use your stereo to cut out the cabin speakers (provided you are not running an amp for in cabin). I think on my stereo it's called "amp mute" where it mutes the internal amp of the head unit and just drives the preouts that go to the sub and the tower speakers. Therefore I just control the sub and the tower speaker volume with the head unit. The interior speakers are totally cut out, but usually there is plenty enough ambient noise from the tower speakers. That's your free solution, if that works for you.

The problem if you are trying to run the cabin speakers is that you will want to have the towers louder than the cabin speakers usually.(IMO) I'd like to have a volume controller on the cabin speakers, but without having a separate amp for them that is difficult.

I also run a HSE volume control unit that make the sounds speed sensitive, but that doesn't change your set up much.

The Kicker ZXM-RLC will allow independant volume control for two zones. The ZXM-RLC- is an active design, providing up to 9 volts of line driver output. Remote knobs can be mounted wherever convenient, and will allow the tower system to be quickly turned up and down, as well as another zone, usually the whole in-boat system.

rrw71,
If your souce unit is on the driver's side so that its instantly accessable and you have an independent fader knob then the fader is all you'll need for tower to in-boat zone control. That's rarely the case. Even if your source unit is on the driver's side the fader is generally within a menu that is tedious and slow to use.
There are lots of options ranging from simple level controls to remote controlled line drivers to an EQ. But the best option is going to be dictated by a number of factors including but not limited to your particular source unit, which side of the boat its on and where your amplifiers are located. If your source unit and amplifier are both on the passenger side then you definitely do not want to add 35 feet of to and from RCA cables in order to utilize a passive level controller.
So fill in the blanks on those details and a best solution will be easy.

The Kicker ZXM-RLC will allow independant volume control for two zones. The ZXM-RLC- is an active design, providing up to 9 volts of line driver output. Remote knobs can be mounted wherever convenient, and will allow the tower system to be quickly turned up and down, as well as another zone, usually the whole in-boat system.